Because who makes the telcos equipments (MSC, HLR, BSC, etc.) can spy a lot and at large scale.
There is a concern of allowing Huawei equipments in telco operator networks.
Would you please stop posting unsubstantive comments to Hacker News? You've been doing it a lot, and that's not cool.
I get that you're frustrated by the comments that appear here on China, but it's not ok to respond by making the site worse. The skew here follows from the fact that the community, while international, is mostly Western. Therefore it reflects Western perspectives. Being in a minority position makes it tempting to lash out against the majority, but doing that breaks the site guidelines and just reinforces the imbalance. What's helpful, if you want to, is to provide a different perspective in a neutral, factual way.
To be fair, the old guard probably did the same before (post 2nd world war). Lucent (US), Nortel (Canada), Siemens (Germany), Alcatel (France) were all too happy to put their equipments in South America, Asia and Africa. It is a fair game, nothing to do with demonizing China. One would expect China to not accept Lucent equipments in its core telco network for the exact same reasons.
Nobody really cared when western countries were spying after the 2nd world war (it was mostly phone calls and network security was a joke). Now, it is digital, and our footprint is way larger. Spying on telco network brings way more information now, than before. It is taken more seriously. Either intentionally (e.g. room 641A: https://en.wikipedia.org/wiki/Room_641A) or not, tapping on telco networks is the great way to spy at scale.
